1. Field of the Invention
The present invention relates to silicon crystallization, and more particularly, to a silicon crystallizing device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing non-uniform crystallization of silicon due to variation of a focal length caused by temperature variation of a projection lens which determines a laser beam pattern.
2. Discussion of the Related Art
With development of information technology, flat display devices in various forms are in great demand. For example, LCDs (liquid crystal displays), PDPs (plasma display panels), ELDs (electro luminescent displays), VFDs (vacuum fluorescent displays), have been researched and developed as monitors for various apparatuses. Recently, the LCDs have attracted considerable attention because of good picture quality, light weight, thin profile, and low power consumption. As a substitute for a CRT (cathode ray tube), the LCD may be utilized as a mobile display for a notebook computer, a television receiver set for receiving and displaying a broadcasting signal, or the like.
Such an LCD includes a liquid crystal panel for displaying a picture, and a driving unit for applying a driving signal to the liquid crystal panel, wherein the liquid crystal display panel is provided with two glass substrates: a TFT array substrate and a color filter substrate, and a liquid crystal layer sandwiched between the two glass substrates.
On the TFT array substrate, a plurality of gate lines are arranged at regular intervals in one direction, a plurality of data lines are arranged perpendicular to the gate lines at regular intervals, a plurality of pixel electrodes are respectively formed on pixel regions defined by the gate lines and the data lines crossing each other in a matrix form, and a plurality of thin film transistors are formed to switch in response to a gate line signal for transmission of a data line signal to each pixel electrode. On the color filter substrate, a black matrix layer is formed for shielding a light incident on parts excluding the pixel regions, R, G, B color filter layers are formed for expressing colors, and a common electrode is formed for reproducing a picture. Moreover, the two glass substrates are made to have a gap therebetween by means of spacers, and bonded together with a sealant having a liquid crystal injection hole, through which a liquid crystal material is injected into the gap.
A general driving principle of the LCD device employs an optical anisotropy and polarity of liquid crystals. Molecules of the thin and elongated liquid crystals tend to orient, and thus enable to control orientation of the molecules by applying an electric field to the liquid crystals as intended. Therefore, if the orientation of liquid crystal molecules is changed, light is refracted accordingly by the optical anisotropy to express picture information. Currently, the active matrix LCD is the most attractive because of its good resolution and motion picture reproduction capability, in which thin film transistors and pixel electrodes connected thereto are arranged in a matrix form.
In the LCD, a semiconductor layer of the thin film transistors is formed of polycrystalline silicon (polysilicon). The thin film transistors and the driving circuit may be formed on the same substrate, which can dispense with a step for connecting the thin film transistors and the driving circuit, thereby simplifying a fabrication process. Moreover, since the polysilicon has a field effect mobility greater than amorphous silicon by 100 to 200 times, the polysilicon has a fast response speed and is stable to temperature and light.
The polysilicon may be formed either by a low temperature process or by a high temperature process. Because the high temperature process is disadvantageous in that a process temperature, which is about 1000° C., for example, and higher than a deformation temperature of an insulating substrate, requires use of an expensive quartz substrate. The quartz substrate has a heat resistance higher than the glass substrate, the low quality crystal with poor surface roughness, and fine crystal grain size that leads to have a poorer applicability than the polysilicon formed by the low temperature process. Research and development are conducted to deposit amorphous silicon at a low temperature and crystallize it to form the polysilicon.
The low temperature process includes laser annealing, metal induced crystallization, and the like. In the laser annealing, a pulse form of a laser beam is directed to the substrate to repeat melting and solidification in 10˜102 nano seconds, thereby minimizing damages to a lower insulating substrate. A related art silicon crystallizing device will be described with reference to the attached drawings, in which a laser beam is directed to a substrate to progress crystallization of the substrate.
FIG. 1 is a graph illustrating a relationship between amorphous silicon grain size and laser energy intensity. As shown in FIG. 1, crystallization of the amorphous silicon may be divided into first, second, and third regions according to intensities of the laser energy.
The first region is a partial melting region in which the laser beam is directed to the amorphous silicon layer at an intensity of laser energy only to melt a surface of the amorphous silicon layer, thereby forming fine crystal grains in the surface of the amorphous silicon layer through partial melting and solidification. The second region is a near-complete melting region in which the laser beam is directed to the amorphous silicon layer at an intensity of laser energy higher than the first region to melt almost all the amorphous silicon layer, thereby obtaining crystal grains grown more than the first region by growing crystals using fine nuclei remained after the melting as seeds. However, it is difficult to obtain uniform crystal grains. The second region has a width substantially smaller than the first region. The third region is a complete melting region in which the laser beam is directed to the amorphous silicon layer at an intensity of laser energy higher than the second region to melt all the amorphous silicon layer, enough to solidify to progress homogeneous nucleation, leading to obtain a crystalline silicon layer of fine and uniform crystal grains.
In the process of forming the polysilicon, the number of applications and an overlap ratio of the laser beam with an energy intensity of the second region are controlled to form uniform and coarse crystal grains. However, the polysilicon shows some problems in that boundaries of the plurality of crystal grains of the polysilicon impede current flow, making fabrication of a reliable thin film transistor device difficult, the collision currents are formed from the collision between electrons in the plurality of crystal grains, and the degradation damages an insulating film, resulting in defects in products. In order to resolve these problems, research and development are conducted to form single crystal silicon by using an SLS (sequential lateral solidification) technology in which a silicon crystal grain grows from a boundary surface of liquid silicon and solid silicon perpendicular to the boundary surface (Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956˜957, 1997). In the SLS technology, by appropriately controlling intensity, area of application and a translation distance of the laser beam, to grow a silicon crystal grain in a lateral direction by a predetermined length, amorphous silicon can be crystallized to single crystal larger than 1 μm.
A laser applying device for the SLS process cannot convert an amorphous silicon layer on a large area of a substrate into the polysilicon at a same time, because the device focuses the laser beam onto a small area. For this reason, after a substrate having the amorphous silicon layer is loaded on a stage such that a laser application position on the substrate may be changed, the laser beam is applied to the entire area of the substrate by applying the laser beam to a predetermined area of the substrate and then moving the substrate to apply the laser beam to the next area.
FIG. 2 is a view schematically illustrating a related art SLS laser applying device. As shown in FIG. 2, the related art SLS laser applying device includes a laser emitting device (not shown) for emitting a laser beam, an attenuator 1 for controlling an intensity of laser beam energy to attenuate the intensity, a first mirror 2a for changing a path of the laser beam, a telescopic lens 3 for diverging the laser beam, a second mirror 2b for changing the path of the laser beam again, a homogenizer and condenser lens 4 for homogenizing and converging the laser beam, a third mirror 2c for deflecting and changing the path of the laser beam, a field lens 5 for appropriate change of a form of the laser beam to a level enough to incident on a laser beam pattern mask, a laser beam pattern mask 6 having a pattern for passing the laser beam in a predetermined pattern, a projection lens 7 for converging the laser beam from the laser beam pattern mask 6 at a converge ratio and applying to the substrate, and a stage 8 for holding a substrate 10 having amorphous silicon to be crystallized deposited thereon.
The laser emission device is an Excimer Laser of 308 nm XeCl or 248 nm of KrF, which emits an unprocessed laser beam passing the attenuator 1, the mirrors 2a, 2b and 2c, the lenses 3-5 and 7 of various functions, and the laser beam pattern mask 6, to incident on the substrate 10 in a predetermined laser beam pattern. The mirrors 2a, 2b, and 2c are provided to prevent the SLS laser applying device from being elongated in one direction and to use a space effectively. If necessary, the number of the mirrors may be adjusted to increase or decrease the space the SLS laser applying device occupies. Though not shown, there is a fastening means for fastening the stage 8, and a moving means for moving the stage 8. That is, in order to crystallize the entire area of the substrate 10, the laser beam is applied to the substrate 10 while fine movements of the stage 8 in an X-axis, and Y-axis directions are conducted to gradually expand the crystallized area.
FIG. 3 is a graph illustrating a relationship between temperature variation and focal length variation measured at the projection lens 7 in FIG. 2. The projection lens 7 has a plurality of lenses of quartz with 99% or more transmittivity arranged in a shape of a barrel. The projection lens 7 has a predetermined pattern of the laser beam incident thereon through the laser beam pattern mask 6 (of FIG. 2), converges the pattern of the laser beam, and directs to the substrate 10 (of FIG. 2). However, as shown in FIG. 3, since the projection lens 7 is involved in linear elongation of a focal length according to a temperature, it is required that no deviation of the focal length be generated at the time of crystallization by maintaining the projection lens 7 at a predetermined temperature in the crystallization when the laser beam pattern is applied in an order of μm level.
Particularly, as shown in FIG. 3, since a deviation of the focal length in a range of ±20˜40 μm is generated by a temperature difference of ±1° C. at an X° C. at which there is no deviation of the focal length, it is required to warm-up the projection lens 7 up to a preset temperature before the crystallization is progressed so as to obtain regular crystallization characteristics. For an example, when the projection lens 7 at the time of loading/unloading the substrate 10 has a temperature lower than a temperature of the projection lens 7 at the time of regular crystallization, the focal length may become shorter during the time the substrate 10 is unloaded from the stage 8 (of FIG. 2) after the crystallization, or the substrate 10 is loaded on the stage 8 before the crystallization,
If the crystallization is made before the projection lens 7 is restored to the regular crystallization temperature after the loading of the substrate 10, the regular crystallization cannot be realized due to incorrect focal length at an initial stage. The related art silicon crystallizing device is not provided with an element, which can prevent an unintended formation of a beam overlap area on the substrate 10 having the crystallization completed at the time when the laser beam pattern is applied through the projection lens 7 before unloading of the substrate 10 and after the crystallization of the substrate 10.
The laser beam pattern mask 6 is provided with a pass through portion for passing the laser beam, and a shielding portion for shielding the laser beam. A width of the pass through portion defines a length of lateral growth of a grain for one time of exposure. Also, the laser beam pattern mask 6, and the area of the substrate 10 having the laser beam applied thereto through the laser beam pattern mask 6 will be described in detail, with reference to the attached drawings.
FIG. 4 is a plan view illustrating the laser beam pattern mask 6 for application of a laser beam, and FIG. 5 is a view illustrating a crystallized area formed by one time application of the laser beam by using the laser beam pattern mask 6 in FIG. 4. As shown in FIG. 4, the laser beam pattern mask 6 is provided with a pass through portion ‘A’ having a pattern opened at first intervals ‘a’, and a shielding portion ‘B’ having a pattern shielded at second intervals ‘b’.
The laser beam is applied by using the laser beam pattern mask 6 as follows. The laser beam is applied, for one time, to an upper portion of a substrate having an amorphous silicon layer deposited thereon through the laser beam pattern mask 6. Herein, the laser beam passes through a plurality of pass through portions ‘A’ in the laser beam pattern mask 6, and melts portions 22 (in FIG. 5) of the amorphous silicon layer onto which the laser beam is incident to liquefy the portion in correspondence to the pass through portions ‘A’. The intensity of the laser energy used in this case is a complete melting region in which the portions of the amorphous silicon layer, having the laser beam applied thereto, completely melts.
A region of the substrate having the laser beam applied thereto by one time of application of the laser beam in correspondence to a region of the laser beam pattern mask having a plurality of successive pass through portions ‘A’ (that is, a region defined by a width L and a height S) is called as a unit region 20 as shown in FIG. 5. After application of the laser beam, growth of silicon grains is progressed from boundary surfaces 21a and 21b of the amorphous silicon region and a liquid silicon region melt completely toward laser applied regions in a lateral direction. The lateral growth of the grains 24a and 24b is perpendicular to the boundary surfaces.
If a width of the portion 22 having the laser beam applied thereto in correspondence to the pass through portion ‘A’ is shorter than a growth length of the crystallized silicon grain 24a, the opposite grains, grown inwardly from and perpendicular to opposite interfaces 21a and 21b of the amorphous silicon region and the portion 22, collide at a middle point (grain boundary 25) to stop the grain growth.
Next, in order to grow more silicon grain, the stage having the substrate placed thereon is moved, and the laser beam is applied to a region next to the laser beam applied portion, to form crystals continuous from the crystals formed at the first time laser beam application. Similarly, the laser beam applied portion melt completely and instantly at the time of the laser beam application has the grain growth from opposite sides in a lateral direction. In general, a length of the grain growth continuous from an adjacent laser beam applied portion progressed as a general laser beam application process is fixed according to widths of the pass through portion “A”, and the shielding portion “B” of the laser beam pattern mask.
The foregoing related art silicon crystallizing device has the following problems. The variation of the focal length of the projection lens with a temperature in the related art silicon crystallizing device leads to require a warming up to a predetermined level at the time of crystallization when the laser beam application is required at a precision in an order of a few μm before progressing crystallization. Therefore, as a method of the warming up, a dummy laser shot may be made before the crystallization to elevate the temperature of the projection lens to a level higher than a predetermined level. However, this causes a problem of reduced productivity in that a laser beam application time period is reduced, or the glass having the dummy laser shot applied thereto cannot be used.
Alternatively, the variation of the focal length of the projection lens with a temperature is predicted in advance, to compensate for the variation of the focal length by changing a distance (Z-axis) between the projection lens and the substrate as much as a deviation. However, this requires a very complicate silicon crystallizing device.